The present invention relates to a radiation pyrometer useful for the measurement of the temperature of a radiating body. More particularly, the present invention relates to a radiation pyrometer that detects and compensates for emissivity that changes with wavelength, as in metals. Additionally, the present invention relates to a radiation pyrometer that enhances the resolution and repeatability of the measured temperature of the radiating body. Additionally, the present invention relates to the technique utilized to enhance the resolution and repeatability of the measured temperature.
Radiation pyrometers are known and commercially available. Typically, pyrometers are used to generate a measured temperature of a radiating body. The term xe2x80x9ctargetxe2x80x9d is used to indicate the radiating body evaluated for temperature determination, and the term xe2x80x9cmeasured temperaturexe2x80x9d is used to indicate the value generated by a pyrometer or a pyrometric technique. The measured temperature may, or may not, be the actual temperature of the target.
Pyrometers are particularly useful for measuring target temperatures when the target is positioned in a remote location, or when the temperature or environment near the target is too hostile or severe to permit temperature measurement by other, more conventional, means or when the act of measuring in a contact manner may itself perturb the target temperature. The terms xe2x80x9cmeasuringxe2x80x9d and xe2x80x9cmeasurexe2x80x9d are use to include all aspects of a pyrometric technique including, but not limited to, energy collection, correlation, data manipulation, the report of the measured temperature, and the like.
Current pyrometers are one of two types: brightness or ratio devices. Brightness and ratio pyrometers both utilize a solution of a form of the Planck Radiation Equation to calculate the target""s measured temperature. The Planck Radiation Equation for spectral radiation emitted from an ideal blackbody is                               L          λ                =                                                            2                ⁢                                  hc                  2                                                            λ                5                                      ⁡                          [                                                ⅇ                                                            hc                      /                      λ                                        ⁢                                          xe2x80x83                                        ⁢                                          K                      B                                        ⁢                    T                                                  -                1                            ]                                            -            1                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
where Lxcex=radiance in energy per unit area per unit time per steradian per unit wavelength interval,
h=Planck""s constant,
c=the speed of light,
xcex=the wavelength of the radiation,
kB=Boltzmann""s constant, and
T=the absolute temperature.
For non-blackbodies,                               H          λ                =                              ϵ            ⁢                          xe2x80x83                        ⁢                          L              λ                                =                      ϵ            ⁢                          xe2x80x83                        ⁢                                                                                2                    ⁢                                          hc                      2                                                                            λ                    5                                                  ⁡                                  [                                                            ⅇ                                                                        hc                          /                          λ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  K                          B                                                ⁢                        T                                                              -                    1                                    ]                                                            -                1                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          2                )            
where Hxcex=the radiation emitted, and
xcex5=emissivity.
In the brightness method of pyrometry, Hxcex and xcex5 are measured at a known wavelength, xcex, and, therefore, T can be calculated.
Brightness devices rely upon capturing a known fraction of the radiation from a source in a particular solid angle. Brightness pyrometers known in the prior art are dependent upon knowing the emissivity of the target, as required by Equation 2, supra. Emissivity is the ratio of the radiation emitted by the target to the radiation emitted by a perfect blackbody radiator at the same temperature. Typically, emissivity is unknown or estimated to a low degree of accuracy. Additionally, the emissivity is often a function of the target temperature, wavelength of radiation examined, and history of the target. This limits the utility of brightness pyrometry severely.
In practice, it is left to the user of a brightness pyrometer to estimate target emissivity, usually based upon an analysis of the target""s composition. The user must then determine if the target""s thermal and environmental history have not appreciably altered the target emissivity. The wavelength or group of contiguous wavelengths of radiation examined are determined by the instrument used, and no selection is possible. It is then left to the user to decide whether or not the indicated target temperature is correct.
Brightness pyrometers are also susceptible to effects of the environment. The gases given off by the target or otherwise present in the atmosphere can selectively absorb radiation at various wavelengths, thus altering the energy reaching the pyrometer and hence the measured temperature. Again, current instruments give no guidance or assistance to the user in surmounting this obstacle.
Ratio pyrometers depend upon graybody behavior. A graybody is an energy radiator which has a blackbody energy distribution reduced by a constant, known as the emissivity, throughout the wavelength interval examined. Ratio pyrometers detect the radiation intensity at two known wavelengths and, utilizing Planck""s Equation, calculate a temperature that correlates to the radiation intensity at the two specified wavelengths.
One form of the Planck Radiation Equation useful for ratio pyrometry is expressed as                     T        =                                            C              xe2x80x2                        ⁡                          (                                                1                  /                                      λ                    1                                                  -                                  1                  /                                      λ                    2                                                              )                                                          ln              ⁢                              xe2x80x83                            ⁢              R                        -                          5              ⁢                              xe2x80x83                            ⁢                              ln                ⁡                                  (                                                            λ                      2                                        /                                          λ                      1                                                        )                                                      -                          ln              ⁡                              (                                                      K                    1                                    /                                      K                    2                                                  )                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          3                )            
where T=absolute temperature;
xcexi=specific wavelength chosen,
Cxe2x80x2=second radiation constant=hc/kB;
R=ratio of radiation intensity at xcex1, to that at xcex2; and
Ki=instrument response factor at each wavelength chosen.
Here the low-temperature, short-wavelength approximation has been made; i.e., [ehc/xcexkBTxe2x88x921] has been replaced with [ehc/xcexkBT].
Tradeoffs must be made in the design of ratio pyrometers, particularly in the wavelengths selected for inspection. Planck""s Equation yields higher precision when the selected wavelengths are further apart. However, broadly spaced wavelengths permit extreme errors of indicated temperatures for materials that do not exhibit true graybody behavior. In practice, the two distinct wavelengths are typically chosen close together to minimize target emissivity variations, and the resulting diminution of accuracy accepted as a limitation of the pyrometric device.
Ratio devices are also affected by gaseous absorptions from the workpiece or environment. If a selective absorption occurs for either of the two wavelengths fixed by the instrument, the measured temperature will be incorrect.
Both brightness and ratio devices are therefore critically dependent on target emissivity and atmospheric absorptions in the region under study.
There is another, more subtle error to which both brightness and ratio devices are prone. If the measuring device has a significant bandwidth at the wavelengths utilized, a simple emissivity correction will not suffice for a target with spectral variation of emissivity. The emissivity correction is treated as a variable gain for both classes of devices (brightness and ratio), and is therefore a linear correction. If the bandwidth is large the contribution from neighboring wavelengths of different emissivity will render the resulting radiation intensity variation with temperature non-linear, since the Planck function is non-linear. This implies that there is no single emissivity correction for certain targets if the bandwidth is large. Furthermore, if any element in the optical path has a spectral transmission dependence, the same error applies; no single gain factor can correct for such an optical element (e.g., a gaseous, absorbing atmosphere, a glass window or lens, a mirror, etc.)
Experimenters have investigated multi-wavelength pyrometry for some time. G. A. Hornbeck (Temperature: Its Measurement and Control in Science and Industry, 3 (2), Reinhold, New York, 1962) described a three-wavelength device that could measure temperatures independent of target emissivity if the emissivity variation was linear over the wavelengths examined. The works of Cashdollar and Hertzberg (Temperature: Its Measurement and Control in Science and Industry, 5 453-463, American Institute of Physics, New York, 1982; U.S. Pat. No. 4,142,417) describe temperature measurement of particulate matter and gas in coal dust explosions using six-wavelength and three-wavelength devices utilizing a least squares fit to Planck""s Radiation Equation under the assumption that the particles are essentially graybodies and that the dust cloud is optically thick.
Gardner et al. (High Temperature-High Pressures, 13, 459-466, 1981) consolidated the contents of a series of papers on the subject. Gardner extends the concept of Hornbeck as well as the work of Svet (High Temperature-High Pressures, 11, 117-118, 1979), which indicated that emissivity could be modeled as linear over a range of wavelengths for a number of materials. Also of interest is a previous publication by Gardner (High Temperature-High Pressures, 12, 699-705, 1980), which discusses coordinate spectral pairs of measured intensity and the associated wavelength. Differences between all possible pair combinations are calculated, and the target emissivity estimated. Use of the emissivity with measured intensities permits calculation of the target temperature. The work of Andreic (Applied Optics, Vol. 27, No. 19, 4073-4075, 1988) calculated the mean color temperature from many spectral pairs and determined that detector noise of only 1% would produce intolerable effects on measurement accuracy. The references of Hornbeck, Cashdollar, Hertzberg, Gardner, Svet, and Andreic, discussed above, are incorporated herein by reference.
In contrast, the present invention measures the radiation intensity at numerous wavelengths of extremely narrow bandwidth to generate a large number of coordinated data pairs of primary data points, fits the primary data points to a mathematical function, generates a statistically significant number of processed data points from the mathematical function, calculates an individual two-wavelength temperature for several pairs of processed data points, inspects the results for internal consistency, and if internal consistency is found, numerically averages the appropriate ensemble of individual two-wavelength temperatures to generate the measured temperature. If internal consistency is not found, that is, if no consensus of temperature is observed in the calculated temperatures, the current invention analyzes the calculated temperatures for emissivity changing with wavelength. Detecting a constant change of emissivity with wavelength, it corrects the calculated temperatures for this change and inspects the results for internal consistency, numerically averages the appropriate ensemble of individual two-wavelength temperatures to generate the measured temperature. A data point is defined as a wavelength and its associated (spectral)intensity such that if each were substituted into Equation 1 a unique temperature would result. A processed data point is a data point as described above except that the spectral intensity is generated by the invention""s mathematical function. A pair of processed data points, hereafter known as a generating pair, is required to generate a temperature by the use of Equation 3, the formula for ratio pyrometry. A substantial portion of;Applicant""s invention was previously disclosed in U.S. Pat. No. 5,772,323, incorporated herein by reference.
Nothing in the prior art envisions generating a non-Planckian mathematical function to fit primary data points, the calculation of multiple processed data points, and the numerical averaging of the multiple processed data points to generate a measured temperature of extreme accuracy and precision with an associated tolerance; or if the associated tolerance is large, indicating the measured temperature is not tightly bounded, further analysis detects and corrects for emissivity changing with wavelength. In contrast to the limited capabilities of previous techniques, the present invention has demonstrated an accuracy of measured temperature to xc2x15xc2x0 C. at 2500xc2x0 C., or xc2x10.15%, with a reproducibility of xc2x10.015%.
It also yields a tolerancexe2x80x94a measure of accuracy for the indicated temperaturexe2x80x94which has never been offered before. It is an extremely useful feature., in that its result is that the user immediately knows to what degree the measurement just made is to be relied upon. This is in stark contrast with prior practice. The accuracy of pyrometers is typically specified by their manufacturers. This specification means that when the target is a blackbody (or possibly a graybody) and the environment does not interfere, the instrument will return a measurement of the specified accuracy.
But measurements of real interest occur with targets and environments of unknown characteristics. The current invention detects whether the target or the environment are not well behaved. In the case of the target this can mean exhibiting other than graybody behavior; in the case of the environment this might result from other than gray or neutral density absorption. In spite of such deficiencies, the present invention extracts the correct temperature. The tolerance reported with the temperature indicates how successful that extraction was.
The present invention also has a unique advantage with respect to immunity from noise. As has been previously described, one reason to choose the wavelengths close together for ratio temperature measurement is to eliminate the variation of emissivity as a contributing factor to the measurement error. The rationale is that if the wavelengths are close together the change in emissivity is likely to be small. However, choosing the wavelengths close together maximizes the effect of noise. The magnitude of the noise generally remains constant throughout the spectrum. Choosing the wavelengths close together insures that the intensity will not differ much between the two wavelengths, thus making the noise contribution a larger fraction of the measured signal.
The invention overcomes this problem by using the weight of the entire spectrum collected to fix each processed-intensity data point. Thus processed data points can be chosen arbitrarily close together without magnifying the noise contribution. Observation and modeling show that the contribution of noise is actually less than that expected from evaluating the expression for error for the extremes of wavelength measured. The error associated with any two wavelength/intensity pairs can be calculated using differential calculus if the error is small:                                           ⅆ            T                    T                =                              T                          C              xe2x80x2                                ⁢                      xe2x80x83                    ⁢                                    (                                                λ                  1                                xc3x97                                  λ                  2                                            )                                      (                                                λ                  1                                -                                  λ                  2                                            )                                ⁢                      xe2x80x83                    ⁢                                    ⅆ              R                        R                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          4                )            
where dR=error in the ratio, and
R=ratio of intensities at two wavelengths.
Here the term dR/R can be replaced with the infinitesimal, xcex94R/R, where xcex94R is the error in the ratio, and similarly, dT/T can be replaced with xcex94T/T where AT is the error in temperature. The equation thus becomes:                                           Δ            ⁢                          xe2x80x83                        ⁢            T                    T                =                              T                          C              xe2x80x2                                ⁢                                    (                                                λ                  1                                xc3x97                                  λ                  2                                            )                                      (                                                λ                  1                                -                                  λ                  2                                            )                                ⁢                      xe2x80x83                    ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              R                        R                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          5                )            
Equation 5 can be used to calculate the maximum expected error, which can be compared to the error actually observed. The observed error of the invention has uniformly been smaller than the calculated value. Equation 5 further points out that the accuracy observed to date is not the limit of the accuracy that can be expected. The invention is calibrated according to a source of radiant intensity, instead of a standard source of temperature. Therefore, if the total error in radiant intensity, xcex94R/R, is reduced to 1%, the expected error at 2500xc2x0 C. is xc2x10.10%.
If the target exhibits graybody behavior in any spectral region, it is also possible for the present invention to quantify the target emissivity in all regions. That is, the spectral emissivity for the entire wavelength range of the data can be quantified once the temperature is known. Further, if the target exhibits non-graybody behavior as is seen in un-oxidized metals such as aluminum, copper, iridium, molybdenum, tantalum, titanium, tungsten, etc. in any spectral region, it is also possible for the present invention to quantify the target emissivity in all regions. That is, the spectral emissivity for the entire wavelength range of the data can be quantified once the temperature is known. Once quantified, changes in emissivity can identify changes in the target as a function of various external effects (time, temperature, chemistry, etc.), as well as identify changes in the target environment, such as off-gassing, reactions, or material decomposition.
In addition, the choice of a source of radiance as the calibration standard extends the useful operating range of the present invention well above currently available temperature calibration standards. Current pyrometers are calibrated by exposing their optical inputs to blackbody sources at the temperature desired and in some fashion (electrical or mechanical) forcing the output of the pyrometer to agree with the blackbody temperature. The limit for such a direct temperature calibration is 3000xc2x0 C., the highest temperature a blackbody source can currently attain reproducibly. The invention described herein, by way of contrast, need only be calibrated by a source of radiant intensity (that is, a device whose emitted radiation is known as a function of wavelength, such as a standard lamp) to yield accurate temperatures. There is no need to expose the invention to the range of desired temperatures for it to be capable of measuring that range, a feature not possible using the prior art.
The present invention is a method to measure the temperature of a radiating body, and a device which utilizes the method.
The measurement of temperature is a problem in many process industries: aluminum, iron and steel, ceramics, cement, glass, and composites are a few examples. Non-contact, and therefore non-perturbing, techniques of radiation pyrometry would be preferred but for the weakness that, as currently practiced, they require knowledge of the target""s emissivity. This parameter is defined as the ratio of the radiation emitted by the sample to that of a blackbody (ideal) radiator at the same temperature.
Unfortunately emissivity is a function of the target""s composition, morphology, temperature, and mechanical and thermal histories, and of the wavelength at which the measurement is made. For some materials, it changes while the temperature measurement is being made. Prior to the present invention, this central difficulty has proven so intractable that the growth of radiation pyrometry has been stunted.
The effect of this difficulty is to preclude trustworthy temperature determination without allowance for emissivity within the measurement. The historically recommended method of accomplishing this is to encase the experiment in a blackbody cavity, thereby allowing the radiation to come to thermal equilibrium. Clearly this is not a practical solution.
The commercially available technique of ratio, or two-color, pyrometry attempts another approach: canceling the emissivity by dividing two measurements of the radiation emitted and calculating the temperature from this ratio. This works in principle but there is still no guarantee that the emissivity is constant at the wavelengths chosen. This concern is the basis for the instrument maker""s dilemma: whether to opt for emissivity cancellation or precision. Emissivity cancellation and precision are mutually exclusive in a ratio instrument, and the choice is signaled by the distance between wavelengths. The closer the wavelengths the more likely the emissivities are to cancel; the farther apart the larger the magnitude of the resultant signal, and thus the greater the precision.
The present invention, which is suitable for measuring the temperature of any radiating body that is above ambient temperature, quantifies radiation intensity at multiple wavelengths, generates a mathematical function to fit the primary data points, calculates multiple processed data points using the mathematical function, utilizes multiple pairs of the processed data points to calculate individual two-wavelength temperature estimates, inspects the results for internal consistency, and numerically averages the estimates to generate a measured temperature of great accuracy and a tolerance, which is a quantification of that accuracy. The invention also permits evaluation of the quality of the emission spectra being measured, and identifies whether the target exhibits true graybody behavior and, if it does not, determines if metallic non-graybody behavior is present, and, if so, corrects for it. When metallic non-graybody behavior is detected, the present invention quantifies radiation intensity at multiple wavelengths, generates a mathematical function to fit the primary data points, calculates multiple processed data points using the mathematical function, utilizes multiple pairs of the processed data points to calculate individual two-wavelength temperature estimates, inspects the results for internal consistency, uses the non-consistent individual two-wavelength temperatures to determine the emissivity""s departure from a constant value, corrects the individual two-wavelength temperatures for this changing emissivity, and numerically averages the estimates to generate a measured temperature of great accuracy and with a tolerance.
The present invention""s ability to quantify radiation intensities at multiple wavelengths with a single sensor minimizes temperature measurement errors due to variations between sensors. Removing this source of intrinsic error permits statistical manipulation of the collected data to enhance the accuracy and reproducibility of the temperature measurement technique. Fitting the primary data points to a mathematical function accommodates target deviations from true graybody behavior, as well as further minimizing the effects of thermal, detector, and instrument noise.
The present invention provides a process for measuring temperature, comprising quantifying the radiation intensity emitted by a radiating body at no less than 4 distinct wavelengths; generating a mathematical function which correlates the radiation intensities to the corresponding wavelength at which the radiation intensity was quantified; and generating a temperature value utilizing Equation 3 and no less than two processed data points generated utilizing the mathematical function. The invention may also be practiced using three or more processed data points generated utilizing the mathematical function. The invention also encompasses the use of only quantified radiation intensity which exhibits emission spectra consistent with known thermal radiation effects for generation of the mathematical function. Data may be said to be consistent when the processed data points are computed at wavelengths where the fractional residuals of the quantified radiation intensity exhibit an RMS value substantially equal to zero or where the quantified radiation intensity exhibits magnitudes of fractional residuals no less than xe2x88x920.1 and no more than 0.1, preferably no less than xe2x88x920.05 and no more than 0.05, most preferably no less than xe2x88x920.02 and no more than 0.02. The invention may also be used to determine the emissivity of the radiating body, as well as the absorption of the intervening environment between the radiating body and the device utilized to quantify the radiation intensity of the body. Additionally, the chemical species present in the environment between the radiating body and quantifying device may be identified and measured.
The invention also includes averaging the individual temperature values calculated utilizing Equation 3 and no less than three processed data points, and the determination of the tolerance of the resulting temperature value by calculating the statistical variation of the temperature values calculated utilizing Equation 3 and no less than three generating pairs. One pertinent statistical variation is the determination of the standard deviation of the average of the individual temperature values calculated.
The invention also encompasses a device, comprising an optical input system, a wavelength dispersion device, a radiation transducer, a means for generating a mathematical function to correlate the radiation transducer output to the corresponding wavelengths of incident radiation; and a means for generating a temperature value utilizing Equation 3 and no less than two processed data points generated utilizing the mathematical function, as well as all the other capabilities described herein.
The present invention thus provides a process and apparatus for temperature determination which exhibits improved accuracy, noise immunity, great adaptability to varied temperature measurement situations, and unlimited high temperature response. In addition, the tolerance of the measured temperature is reported, temperature measurements are made independent of knowledge of the target emissivity, and all corrections are made digitally (in a mathematical expression, leaving the hardware completely versatile). These features provide a method and device which are effective in non-ideal, i.e., absorbing or reflecting, environments.
The present invention provides for a process for measuring the temperature of a radiating body, comprising a) quantifying the radiation intensity emitted by a radiating body at no less than 4 distinct wavelengths; b) generating a mathematical function which represents said quantified radiation intensities at the corresponding wavelength at which said radiation intensity was quantified; c) selecting no less than two specific wavelengths; d) generating a spectral intensity using said mathematical function for each of said wavelengths; e) generating an individual two-wavelength temperature value of said radiating body utilizing the radiation equation       T    12    =                    C        xe2x80x2            ⁡              (                              1            /                          λ              1                                -                      1            /                          λ              2                                      )                            ln        ⁢                  xe2x80x83                ⁢        R            -              5        ⁢                  xe2x80x83                ⁢                  ln          ⁡                      (                                          λ                2                            /                              λ                1                                      )                              
where T12=individual two-wavelength temperature,
xcex1, xcex2, . . . xcexn=specific wavelengths selected,
Cxe2x80x2=second radiation constant, and
R=ratio of the generated spectral intensity I1, calculated using said mathematical function at xcex1, to the generated spectral intensity I2, calculated using said mathematical function at xcex2; f) analyzing said individual two-wavelength temperatures for lack of consensus; g) calculating the functional dependence of emissivity on wavelength for said radiating body; h) calculating new individual two-wavelength temperatures using said functional dependence of emissivity on wavelength; and i) reporting the average of said new individual two-wavelength temperatures.
The present invention provides for a temperature measuring device, comprising; a) an optical input system which receives a portion of the emitted radiation of a radiating body; b) a wavelength dispersion device which separates said emitted radiation according to wavelength; c) a transducer which senses said separated radiation and provides a quantified output corresponding to radiation intensity for each wavelength of said emitted radiation; d) means for generating a mathematical function to represent said quantified output of said radiation transducer as a function of wavelengths; e) means for selecting no less than two specific wavelengths; f) means for generating a spectral intensity value at each said selected specific wavelength, utilizing said mathematical function; and g) means for generating an individual two-wavelength temperature value utilizing no less than two said spectral intensity values and the radiation equation       T    12    =                    C        xe2x80x2            ⁡              (                              1            /                          λ              1                                -                      1            /                          λ              2                                      )                            ln        ⁢                  xe2x80x83                ⁢        R            -              5        ⁢                  xe2x80x83                ⁢                  ln          ⁡                      (                                          λ                2                            /                              λ                1                                      )                              
where T12 individual two-wavelength temperature,
xcex1, xcex2, . . . xcexn=specific wavelengths selected,
Cxe2x80x2=second radiation constant, and
R=ratio of the generated spectral intensity I1, calculated using said mathematical function at xcex1, to the generated spectral intensity I2, calculated using said mathematical function at xcex2;
Other advantages will be set forth in the description which follows and will, in part, be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.